The present invention relates to an optical element to be used in a video camera, still video camera, copying machine, and the like and, more particularly, to an optical element having a plurality of reflection surfaces with curvatures.
Conventionally, as a photographing optical system including a reflection surface, for example, a so-called mirror lens system is known, as shown in FIG. 29.
Referring to FIG. 29, object light 174 is converged and reflected toward the object side by a concave mirror 171, and is imaged on an image plane 173. This mirror lens system is based on the arrangement of a so-called Cassegrain reflecting telescope, and aims at a small total lens length by folding the optical path of a telescopic lens system with a large total lens length using two opposing reflection mirrors.
In the objective lens system of a telescope as well, many systems for shortening the total optical length using a plurality of reflection mirrors are known in addition to the Cassegrain type. That is, the optical path is efficiently folded by using a reflection mirror in a lens system with a large total lens length, thus obtaining a compact optical system.
However, in general, in the Cassegrain reflecting telescope, some object light rays are eclipsed by a concave mirror 172.
This problem arises from the fact that a chief ray 176 of the object light 174 is located on an optical axis 175. In order to solve this problem, many mirror optical systems which separate the chief ray 176 of the object light 174 from the optical axis 175 by using a reflection mirror at a decentered position have been proposed.
As the methods of separating the chief ray of object light from the optical axis, a method using a portion of a reflection mirror which is rotation-symmetric to the optical axis, as disclosed in, e.g., U.S. Pat. Nos. 3,674,334, 4,737,021, and the like, and a method of decentering the central axis itself of the reflection mirror from the optical axis, as disclosed in U.S. Pat. Nos. 4,265,510, 5,063,586, and the like, are available.
FIG. 30 shows an example of U.S. Pat. No. 3,674,334 as an example of the method of using a portion of a rotation-symmetric reflection mirror.
Referring to FIG. 30, a concave mirror 181, convex mirror 182, and concave mirror 183 are originally rotation-symmetric to an optical axis 184, as indicated by the two-dashed chain lines. However, since the concave mirror 181 uses only its portion above the optical axis 184, the convex mirror 182 uses only its portion below the optical axis 184, and the concave mirror 183 uses only its portion below the optical axis 184, the chief ray of object light 185 can be separated from the optical axis 184, and the object light 185 can be output without being eclipsed.
FIG. 31 shows an example of U.S. Pat. No. 5,063,586 as an example of the method of decentering the central axis itself of the reflection mirror from the optical axis.
Referring to FIG. 31, when an axis perpendicular to an object plane 191 is defined as an optical axis 197, the central coordinates and central axes of the surfaces of a convex mirror 192, concave mirror 193, convex mirror 194, and concave mirror 195 are decentered from the optical axis 197, and object light 198 can be efficiently imaged on an image plane 196 without being eclipsed by the reflection mirrors by appropriately setting the decentering amounts and the radii of curvature of the respective surfaces.
In this way, when the reflection mirrors that construct the mirror optical system are decentered, object light can be prevented from being eclipsed. However, since the individual reflection mirrors must be set with different decentering amounts, a structure for attaching the respective reflection mirrors is complicated, and it is very hard to assure high attachment precision.
As one method of solving this problem, for example, when a mirror system is formed as one block, assembly errors of optical parts upon assembly can be avoided. Conventionally, as optical systems having a large number of reflection surfaces as one block, for example, optical prisms such as a pentagonal roof prism, Porro prism, and the like, which are used in camera finder systems, a color-separation prism for separating a light beam coming from a photographing lens into three, red, green, and blue light beams, and imaging object images based on the respective color light beams on the corresponding imaging element surfaces, and the like are known.
The function of a pentagonal roof prism popularly used in a single-lens reflex camera as an example of the optical prism will be explained below with reference to FIG. 32.
Referring to FIG. 32, reference numeral 201 denotes a photographing lens; 202, a quick return mirror; 203, a focal plane; 204, a condenser lens; 205, a pentagonal roof prism; 206, an eyepiece; 207, the pupil of the observer; 208, an optical axis; and 209, an image plane.
Light rays coming from an object (not shown) are transmitted through the photographing lens 201, are reflected upward in the camera by the quick return mirror 202, and are imaged on the focal plane 203 located at a position equivalent to the image plane 209.
Behind the focal plane 203, the condenser lens 204 for imaging the exit pupil of the photographing lens 201 on the pupil 207 of the observer is placed. Behind the condenser lens 204, the pentagonal roof prism 205 for converting an object image on the focal plane 203 into an erected image is placed.
An object image defined by object light that enters the pentagonal roof prism 205 via an entrance surface 205a is horizontally inverted by a roof surface 205b. The object light is then reflected by a reflection surface 205c toward the observer.
The object light reflected toward the observer side is transmitted through an exit surface 205d of the pentagonal roof prism 205, and reaches the eyepiece 206, which converts the object light into nearly collimated light by its refractive power. The nearly collimated light beam then reaches the pupil 207 of the observer, and the observer can observe the object image.
As a major problem of such optical prisms represented by the pentagonal roof prism, harmful ghost light is likely to be produced due to irregular incoming light into the prism from positions and angles other than those of effective light rays.
In the pentagonal roof prism with the above-mentioned structure, ghost light that enters the prism at an angle different from that of effective light rays, as indicated by the arrow in FIG. 32, is reflected in turn by the roof surface 205b and reflection surface 205c, is totally reflected by the entrance surface 205a, and then leaves the prism from the lower portion of the exit surface 205d toward the observer.
If such ghost light is produced, since its number of times of reflection is different from that of normal effective light rays, a vertically inverted image appears on the lower side of the observation frame.
In order to remove the ghost light, a light-shielding groove 200 is formed on the exit surface 205d of the pentagonal room prism 205.
By painting the entire prism surface except for the entrance surface 205a and exit surface 205d in black, a reflection film deposited on the roof surface 205b and reflection surface 205c is protected from environmental changes in, e.g., temperature, humidity, and the like, and light rays coming from outside the prism are intercepted. Since such optical prism has a plurality of reflection surfaces that are integrally formed, the respective reflection surfaces have a very accurate relative positional relationship, and do not require any positional adjustment.
Note that the principal function of such prism is to invert an image by changing the direction the light rays travel, and the individual reflection surfaces are defined by planes.
By contrast, optical prisms, the reflection surfaces of which have curvatures, are disclosed in, e.g., U.S. Pat. No. 4,775,217 and Japanese Patent Laid-Open No. 2-297516.
U.S. Pat. No. 4,775,217 relates to the structure of an eyepiece in an observation optical system. In the structure of this article, as shown in FIG. 33, display light 215 coming from an information display member 211 is reflected toward the object side by a reflection surface 212, and reaches a surface 213 having a curvature that defines a concave surface.
The concave surface 213 converts the display light 215 as divergent light from the information display member 211 into nearly collimated light by its power, and guides it to a pupil 214 of the observer, thus making the observer see a displayed image.
In the structure of this article, an object image can be seen as well as observation of the displayed image.
Object light 216 enters a surface 217 nearly parallel to the reflection surface 212, and reaches the concave surface 213. Since a semi-transparent film, for example, is deposited on the concave surface 213, some light components of the object light 216 are transmitted through the concave surface 213, and some other light components are reflected thereby. The transmitted object light 216 is transmitted through the reflection surface 212 and reaches the pupil 214 of the observer. In this way, the observer can observe the object light 216 and display light 215, which are superposed each other. Also, Japanese Patent Laid-Open No. 2-297516 also relates to the structure of an eyepiece in an observation optical system. In the structure of this article, as shown in FIG. 34, display light 224 originating as collimated light from an information display member (not shown) is transmitted through a flat surface 227, and becomes incident on a parabolic surface 221.
The parabolic surface 221 focuses the display light 224 to form an image on a focal plane 226.
At this time, since the focused display light 224 reaches the focal plane 226 while being totally reflected between the flat surface 227 and a flat surface 228 parallel to this surface 227, a low-profile structure of the entire optical system is realized.
The display light 224 coming from the focal plane 226 as divergent light becomes incident on a parabolic surface 222 while being totally reflected between the flat surfaces 227 and 228. The parabolic surface 222 converts the display light 224 into nearly collimated light and guides it to a pupil 223 of the observer, thus making the observer recognize a displayed image.
In this article, the observer can also see an object image as well as observation of the displayed image by the structure similar to that of U.S. Pat. No. 4,775,217.
Since such optical prisms having reflection surfaces with curvatures normally suffer more optical performance deterioration resulting from decentering of each reflection surface than an optical prism constructed by flat surfaces alone, the allowable positional precision for each reflection surface is very strict. However, U.S. Pat. No. 4,775,217 and Japanese Patent Laid-Open No. 2-297516 do not mention any of the adjustment method, assembly method, manufacturing method, and the like of the respective reflection surfaces to compensate for the positional precision of each reflection surface.
On the other hand, as the number of reflection surfaces of an optical prism increases, the decentering amounts of the respective reflection surfaces accumulate due to aberration correction of the optical prism. Hence, the allowable decentering amount per reflection surface becomes smaller and stricter with increasing number of reflection surfaces. For this reason, a method of accurately compensating for the positional precision of each reflection surface is demanded.
Furthermore, these optical prisms are manufactured by molding using a metal mold to meet recent low-cost requirements.
For example, a pentagonal roof prism, which was conventionally manufactured by polishing a glass block, is formed by molding using a metal mold as a so-called hollow pentagonal prism, in which the reflection surfaces 205b and 205c shown in FIG. 32 are formed by reflection mirrors, and are integrally formed with a hollow prism. Upon forming the hollow pentagonal prism by molding, since the reflection mirrors are formed by flat surfaces alone, the imaging performance of the finder system does not deteriorate irrespective of slight positional deviations of the reflection mirrors.
However, when an optical prism having reflection surfaces with curvatures is formed by molding, a metal mold which assures higher positional precision of each reflection surface than the optical prism constructed by flat surfaces alone is required.
Also, when an optical prism having reflection surfaces with curvatures is formed by molding, a metal mold structure, which can cope with a complicated optical prism integrally formed with a plurality of reflection surfaces with curvatures, which are set at decentered positions, is demanded.
The present invention has been made in consideration of the aforementioned problems, and has as its object to suppress relative decentering of reflection surfaces, which must have highest precision, and to prevent optical performance from deteriorating, in an optical element in which a plurality of reflection surfaces with curvatures are placed and formed adjacent to each other.
It is another object of the present invention to increase the degree of freedom in aberration correction of an optical element, and to improve the imaging performance of the optical element.
It is still another object of the present invention to accurately set the spacing between reflection surface blocks at predetermined positions while facilitating the manufacture of the respective reflection surface blocks.
It is still another object of the present invention to prevent effective light rays in an optical element from being eclipsed.
It is still another object of the present invention to reduce the number of parts, to reduce errors produced upon movement of an optical element, and to prevent effective light rays in an optical element from being eclipsed.
It is still another object of the present invention to obtain a low-cost optical element, which can be formed by molding irrespective of its shape to have reflection surfaces at accurate positions.
It is still another object of the present invention to obtain an optical element which suffers less ghost.
It is still another object of the present invention to allow the directions of light rays that enter and leave an optical element to be set arbitrarily.
In order to solve the above-mentioned problems and to solve the objects, the first aspect of an optical element according to the present invention is characterized by the following arrangement.
That is, there is provided an optical element comprising a first reflection surface block formed by placing a plurality of reflection surfaces having curvatures at neighboring positions, and a second reflection surface block which opposes the first reflection surface block, and is formed by placing one or a plurality of reflection surfaces at neighboring positions, wherein the first and second reflection surface blocks are formed by a metal mold.
Also, the second aspect of an optical element according to the present invention is characterized by the following arrangement.
That is, there is provided an optical element wherein a first reflection surface group including a plurality of reflection surfaces having curvatures placed at neighboring positions, and a second reflection surface group which opposes the first reflection surface group and includes one or a plurality of reflection surfaces having curvatures placed at neighboring positions, are formed on surfaces of a transparent member, and the transparent member is formed by a metal mold.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.